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24 January 2009

BOBGRAM7 issued 25 Jan 2009

WEATHERGRAMYOTREPSIssued 25 JAN 2009 Bob McDavitt's ideas for sailing around the South Pacific.Disclaimer: Weather is a mix of pattern and chaos; these ideas come fromthe patterned world of weather maps, so please fine-tune to your place.Dates are in UTC unless otherwise stated.

We are now entering HIGH SUMMER... the southern hemisphere equivalent of"the Dog Days' of a dawning Sirius that is associated with the hottesttime of the year. This year in Fiji, the return of the sun after twodrenching weeks has caused very humid conditions there indeed. NZ has asuperb example of a summer HIGH stalled over the North Island-- and thusa westerly flow over the South Island --- all set to make this to belikely to be the warmest week of the year for NZ.

TROPICSThe South Pacific Convergence Zone SPCZ is active in a zone from Tuvaluto Samoa along with some westerly winds on its northern side. It hasdumped a lot of rain in a branch from Samoa to Southern Cooks over pastweek, associated with a tropical depression. There was another wetbranch from Samoa to French Polynesia, and a weaker branch from NorthernAustralia across the Coral Sea to Loyalty Islands.

Over Monday and Tuesday, there is a good chance that a low may form overSPCZ and drift south over or just east of Tonga, fading back into atrough from Wednesday 28 Jan. The main rain with this feature will beon its eastern side as it travels south-it may well wander a bit to eastor west as well, so treat with respect.

Another Low may possibly form on SPCZ to North of Fiji - perhaps aroundRotuma - on or around Wed 28 Jan and then take a generally southwesttrack across Vanuatu and onto New Caledonia around 2 Feb. Not sure onthis ... NOGAPS model seems to be overdoing these lows and EC model isperhaps undergoing them for a change, so this is an amalgam of possibleproceedings. Next week/early in Feb I think (along with the EC model)that the focus of the tropical lows will be the Coral Sea.

We are nearing the end of January and haven't had a cyclone named in theSouth Pacific so far this season, but one of the lows mentioned in pastparagraph may become worthy enough.

SUBTROPICAL RIDGE The HIGH stalled in the Tasman Sea / North Island area has already beenmentioned. There will be a High replacement on Tuesday (old high fades,new High builds in South Tasman Sea behind a front that brings a coolingand damp southerly change that will be welcomed by eastern NZ). TheNew HIGH should be a BFH, a big fat high, building to 1030 hPa incentral Tasman Sea on Thursday and lasting there until another Highreplacement occurs around Sat 31 Jan /Sun 1Feb.

There will be a well-defined squash zone of enhanced trade winds betweenthis Tasman HIGH and the low pressures in the Coral Sea. This squashzone is expected to be strongest between New Caledonia and theQueensland coast between 20 and 25S from Wednesday to Friday.

TASMAN SEA/NZThe superb summer High scenario may well make this the sunniest andwarmest week of the year for much of the country (but several wet daysin Fiordland, and some isolated afternoon downpours where sea breezescollide over hot hinterland along with the arrival of some cooler airaloft as surface pressure drops.